1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus including a charging device in which a cleaning member slidably contacts a surface of the charging device so as to clean the surface thereof.
2. Discussion of the Background
In an image forming apparatus, such as a copying machine, a printer, a facsimile machine, or the like, it is desired a photoconductive element, a developing device, a charging device, etc. are compact to reduce the total size of the image forming apparatus.
Many charging devices employ a charging roller, which uniformly charges a surface of the photoconductive element by applying a voltage to the charging roller. Further, the charging roller contacts the photoconductive element. Typically, in such a charging device, a roller cleaning member made of, for example, a sponge or the like, slidably contacts a surface of the charging roller with an appropriate pressure. When the charging roller contacting the photoconductive element is rotated, the roller cleaning member scrapes foreign substances, such as toner and paper powder, from the surface of the charging roller.
Generally there are two types of roller cleaning members. A first type constantly contacts the surface of the charging roller, and is called a "constant contact type roller cleaning member." The second type contacts the surface of the charging roller only when needed, and is called a "contact/separate type roller cleaning member."
One advantage of the contact/separate type roller cleaning member is that when the contact/separate type roller cleaning member contacts the charging roller, the contact/separate type roller cleaning member makes an impact on foreign substances, such as toner and paper powder which adhere to the surface of the charging roller. Thus, because of the impact from the contact/separate type roller cleaning member, the foreign substances are easily dropped from the surface of the charging roller.
However, because the contact/separate type roller cleaning member requires an additional mechanism so it can contact and separate from the surface of the charging roller, the contact/separate type roller cleaning member tends to be large in size. Therefore, because the image forming apparatus is required to be compact, the contact/separate type roller cleaning member is unfavorable.
On the contrary, the constant contact type roller cleaning member does not require an additional mechanism to contact and separate from the surface of the charging roller. Therefore, the constant contact type roller cleaning member is more compact.
However, the constant contact type roller cleaning member does not make an impact on foreign substances which adhere to the surface of the charging roller. Further, when the foreign substances have the property of a glass transition and when image forming operations are performed to more than 5,000 sheets, the foreign substances are likely to be fused on the surface of the charging roller by the frictional heat produced between the charging roller and the roller cleaning member.
When the foreign substances are fused on the surface of the charging roller, the charging roller cannot perform a charging function on the fused parts thereof. As a result, a black line occurs on a sheet at every pitch corresponding to the circumferential length of the charging roller in a so-called negative-to-positive developing method. Therefore, an image quality is significantly reduced.
For the above-described reasons, the constant contact type roller cleaning member is considered to be unfavorable for practical use and is not often used, even though the roller cleaning member can be made more compact.